The present invention relates to a cassette tape recorder for automatic stereo sets.
It is well known from experience that an ordinary cassette tape recorder may produce a high quality sound when a vehicle in which the cassette tape recorder is installed is in a stationary condition but that the same tape recorder may produce a poor quality sound when the vehicle is in motion and is experiencing large up-and-down vibrations. It is also known that, particularly when the remaining amount of tape to be played is large, wow is increased to thereby result in a poor quality sound production.
Various studies of this wow phenomenon have been made by the present inventors. In one experiment, an up-and-down vibration of 1G was applied to an ordinary cassette tape recorder at a changing frequency, and the resulting wow was measured. The results are shown in FIG. 1, which is a graph shoing the wow (RMS) characteristics vs. frequency. As is apparent from the characteristic curve, wow exceeding 1% was generated at a vibration of 40 Hz and also at 100 Hz or more. These characteristics were measured in another way, that is, with auditory sense compensation, and the results were as shown in FIG. 2. The wow generated at the vibration of 100 Hz or more was less than 0.2% and caused no problem, but the wow generated at about 40 Hz was still more than 0.2%, which caused the quality of sound to be degraded.
Detailed analyses of the wow at 40 Hz were then made, which can be understood with reference to the structure of a conventional tape recorder and the tape cassette and the operations thereof as will be explained in detail with reference to FIGS. 3 through 8.
Referring first to FIGS. 3 and 4, a tape cassette case generally designated by reference numeral 1 is made up of cassette halves 1a and 1b and a tape 2 encased therein. The tape 2 is fixed at either end to a pair or reels 3a and 3b by means of respective clampers 4a and 4b, so that the tape can be wound onto the reels 3a and 3b. Thus, when the winding diameter of the tape on one side is increased, the winding diameter of the tape on the other side is decreased. In the drawings, the tape is fully wound on supply reel 3b. It is a typical feature of the tape cassette that no flanges are provided on the reels 3a and 3b, to thereby decrease the distance between reels 3a and 3b, which in turn leads to a miniaturization of the overall physical cassette size. However, without flange members, the tape may be wound on the reels 3a and 3b non-uniformly in a width wise direction. In order to eliminate this defect, i.e., to prevent the tape from contacting with the inner walls of the cassette halves 1a and 1b, and in order to achieve such prevention without causing any substantial additional friction, a space is provided between the tape and each inner wall of the cassette halves 1a and 1b, and a sheet 5a or 5b, which is a so called "retainer", is interposed in this space so that the reels 3a and 3b are retained at a suitable position within the case 1.
A pair of guide rollers 6a and 6b, a pressure pad 7, and the like are encased in the case together with the tape 2 and the reels 3a and 3b. In the walls of the case 1 are formed a pair of capstan shaft insertion holes 8a and 8b, positioning pin insertion holes 9a, 9b, 9c and 9d, a pair of pinch roller insertion holes 10a, 10b, reel shaft insertion holes 11a and 11b, and a magnetic head insertion hole 12. Reference characters 3a' and 3b' denote engagement lips formed at an angular interval of 60.degree. on the circumferential periphery of the reel 3a or 3b.
On the other hand, the cassette tape recorder comprises a pair of reel shafts 20a and 20b, a capstan shaft 21, a pinch roller 23 and a magnetic head 24. When the tape cassette 1 is mounted on the tape recorder, the reel shafts 20a and 20b are inserted through the reel shaft insertion holes 11a and 11b into the interior of the tape cassette 1 and engage the reels 3a and 3b. At the same time, the capstan shaft 21 is inserted into the capstan shaft insertion hole 8a. Then, when the tape recorder is played, the pinch roller 23 is inserted through the pinch roller insertion hole 10a into the interior of the tape cassette 1 to clamp the tape 2 against the capstan shaft 21 so that the pinch roller cooperates with the capstan shaft 21 rotating at a constant speed to move the tape 2 at a constant speed. At the same time, the magnetic head 24 is inserted through the magnetic head insertion hole 12 into the interior of the cassette case 1 to clamp the tape 2 against the pressure pad 7 to thereby reproduce the information recorded on the tape 2.
In the thus described play condition, the reel shaft 20a serves to take up the tape 2 and is driven by a drive source (not shown), whereas the reel shaft 20b serves to feed the tape 2 and is not driven by the source, but instead is adapted to apply a constant back tension to the tape 2.
The reel shaft 20b on the supply side of the conventional construction is shown in FIGS. 5 and 6, wherein the character C denotes a chassis of the cassette tape recorder, from which an upright shaft 20b.sub.1 extends vertically. A reel shaft member 20b.sub.2 having a regular hexagonal cross section is freely rotatably mounted on the upright shaft 20b.sub.1. A cylindrical hub 20b.sub.3 surrounds the shaft 20b.sub.2 so as to be slidable in the axial direction but to positively engage the shaft member 20b.sub.2 with respect to the horizontal or rotational direction. The cylindrical hub 20b.sub.3 is biased to move in one direction by a spring 20b.sub.4 interposed between the bottom surface of the hub and the facing portion of the shaft member 20b.sub.2. The hub 20b.sub.3 is held at a position shown by a retaining member 20b.sub.5 tightly engaged with the shaft member 20b.sub.2.
As described above, the shaft member 20b.sub.2, the cylindrical hub 20b.sub.3, the spring 20b.sub.4 and the retaining member 20b.sub.5 are formed as a unit which is prevented from being pulled apart from the upright shaft 20b.sub.1 by means of a stopping member 20b.sub.6 mounted on the top end of the upright shaft 20b.sub.1, as shown in FIG. 5. A leaf spring 20b.sub.7 interposed between a flanged portion of the upright shaft 20b.sub.1 and the shaft member 20b.sub.2 is provided in order to produce a constant frictional force between the reel shaft and the upright shaft 20b.sub.1 to thereby apply a back tension to the tape 2. With the thus constructed reel shaft 20b, when the tape cassette is inserted, if the reel lips 3b' of the cassette are not properly meshed with the outer projections 20b'.sub.3 of the cylindrical hub 20b.sub.3, the cylindrical hub 20b.sub.3 may be pressed downwardly so that no damage occurs. If a simpler construction is desired, as shown in FIG. 7, projections 20b'.sub.2 may be formed directly on the shaft member 20b.sub.2 with the shaft member rotatably mounted on the upright shaft 20b.sub.1. The leaf spring 20b.sub.7 provides back tension and is also provided with somewhat more bending range.
Now, when the above-described tape recorder is in the play condition, let us asseme that an up-and-down vibration is applied thereto as shown by the arrow in FIG. 4. When the frequency of vibration is very low, e.g. about 1 Hz, the reels 3a and 3b are vibrated together with the case 1 and chassis C. However, when the frequency is increased, the reels 3a and 3b may remain relatively stationary while only the case 1 vibrates vertically together with the chassis C. For this reason, the upper and lower inner walls of the case 1 collide against the reels 3a and 3b with high energy. In such a condition, the tape 2 may be elongated or shortened between the point a on the tape 2 clamped between the capstan shaft 21 and the pinch roller 23 and the point b at the winding end of the reel 3b.
It is to be noted that the tape 2 between the above described points a and b has a resonance at a resonance frequency f.sub.O which is determined according to the spring constant of the tape 2 and the inertial moment of the tape wound on the reel 3b. Now, when the value of f.sub.O is calculated with a typical back tension of 2 to 3 g, a value of f.sub.O =30-40 Hz, is derived.
When the cassette tape recorder is in its play condition, as shown in FIG. 8a, the projections 3b' of the supply reel 3b are engaged with the projections 20b'.sub.3 of the reel shaft 20b so that the projections 3b' rotate the reel shaft 20b while the supply reel is rotated in the direction of the arrow. However, when the above noted resonance is generaged, the rotation of the reel 3b may be stopped by the elongation of the tape 2 as shown in FIG. 8b. Thereafter, due to the reduction of the tape, the reel 3b is rapidly rotated to thereby rotate the reel shaft 20b with high energy and at a higher speed as shown in FIG. 8c. For this reason, a reactive force occurs whereby the shaft projections 20b'.sub.3 push back on the reel projections 3b' so that, with the next elongation of the tape 2, the projections 3b' and 20b'.sub.3 of both members are separated from each other as shown in FIG. 8d. Once such a state is generated, collision and reaction are alternately generated between the projections 3b' and 20b'.sub.3 to thereby cause so-called jitter. As a result, the travelling speed of the tape 2 on the surface of the magnetic head 24 is changed, which causes wow to be generated at about 40 Hz.
Also, in the reel shaft according to the prior art, there would be a radial displacement between the reel and reel shaft of cassette. Therefore, the reel would not rotate coaxially with the reel shaft. This would cause a vertical fluctuation and a resonance of tape so that the reel might be moved in a radial direction of the reel shaft to produce an unstable travel of tape and to increase a wow.